The present invention relates to a generator for producing an alternating current at an ultrasonic frequency for driving an ultrasonic transducer. In particular, the present invention relates to high power ultrasonic generators.
Ultrasonic generators for operating transducer/horn assemblies for various ultrasonic applications such as the welding of plastic parts or the like are well known. Such generators have performed relatively well in low power applications, i.e., when the generator is operating at 800 watts or less of output power and/or uses a power supply voltage of less than 200 VDC. At these lower power levels the currents and voltages utilized within the system are generally well within the limits of available power transistors.
But with the development of ultrasonic applications requiring higher power ultrasonic generators, it has been necessary to utilize higher power supply voltages in the range of 300-400 VDC, derived from a 240 VAC line source. These higher voltages and the resulting higher currents create serious problems when the operation of the system deviates from optimum conditions. Thus, current overloads which result from overloading of the transducer/horn assembly or deviation of the operating frequency or phase thereof from the nominal operating frequency tend readily to burn out the power transistors and other components in the generator. This necessitates either overdesign of the system so as to tolerate the worst-case current loads, or frequent replacement of power transistors, both very expensive solutions. While the prior art systems typically utilize fuses or circuit breakers to de-energize the system in the event of a current overload, these measures are effective only in protecting the user's power lines, and do not operate fast enough to protect circuit components such as power transistors which can burn out in a matter of microseconds. Furthermore, such protective devices have to be reset each time they are tripped.
Another difficulty with the prior art ultrasonic generators is that during start-up heavily loaded massive transducer/horn assemblies tend to draw extremely large currents. Various types of current-limiting arrangements have been utilized in the prior art but have presented significant disadvantages. For example, it is known to limit the direct current flow to the power transistors during start-up of the device, but only partially effective means have been used. Furthermore, while these arrangements tend to protect the power output transistors, they do not protect other parts of the generator, such as the oscillatory components, which also tend to undergo high demand at start-up.
Another transient overload phenomenon which can occur in ultrasonic generators, particularly those using a transistor bridge in the power output circuit, stems from the fact that a transistor has a certain storage time such that the collector-emitter junction will continue to be conductive for a predetermined short time after control voltage has been removed from the base. Thus, it is possible that the conducting conditions of the opposite halves of the bridge may momentarily overlap, thereby creating a short-circuit, and a momentary surge of current through this low impedance path can easily burn out the power transistors. U.S. Pat. No. 3,487,237, issued to V. G. Krenke, discloses the technique of utilizing a saturable reactor in series with a power transistor for introducing a slight delay in current conduction through the transistor, but the saturable reactor is bulky and expensive and is inefficient because it consumes a considerable amount of power which is dissipated by heating of the saturable reactor.
Finally, prior art ultrasonic generators typically utilize a motional feedback signal representative of the frequency and amplitude of transducer vibration for synchronizing the oscillatory circuitry, thereby to maintain the transducer/horn assembly at mechanical resonance for various loading conditions. Since the system, including the transducer/horn assembly, introduces a certain phase shift at no-load conditions, systems such as that disclosed in U.S. Pat. No. 3,432,691 utilize a series resonant circuit in the feedback loop to introduce a counterbalancing phase shift, but such circuitry dissipates considerable energy in the form of heat, which is essentially wasted. It is also known to use bandpass filters in the feedback loop to eliminate unwanted resonances of the transducer/horn assembly but such filters exhibit undesirable frequency-dependent phase shift characteristics. U.S. Pat. No. 4,056,761 discloses a system for achieving the effect of bandpass filtering without the detrimental phase shift. But that system requires the use of a pickup detector on the sonic transducer or horn, necessitating inconvenient mechanical mounting arrangements.